The present invention relates generally to metal matrix composite (MMC) materials, and more particularly to polymer-reinforced metal matrix composites wherein polymer filaments are formed during processing of the MMC.
Metal matrix composite materials, wherein a metal is reinforced with a non-metal, offer various advantages over the metals alone. In particular, MMC's incorporating a less-dense reinforcing non-metal have been developed for use in low-density applications such as for aircraft components. Such MMC's provide other improved properties as well. For instance, the toughness, specific strength, and specific modulus of metals such as aluminum and titanium can be enhanced by reinforcing them with boron, carbon, or silicon carbide filaments. These filaments are prepared separately and then incorporated into the matrix by processes such as vacuum hot-pressing, hot-isostatic pressing, and melt infiltration. Such processes, while yielding a product with many desirable mechanical properties, are often labor intensive and relatively expensive compared to processes for producing the unreinforced metal matrix material. Another commonly-used form of MMC is the laminated or sandwich structure, which is formed by bonding metal sheet or foil to prefabricated fibers. This process tends to limit the form of the product to sheet or plate, and consequently limits subsequent processing to processes which can be performed on sheet or plate and which will not break the fibers.
The low density of polymers makes them an attractive candidate for use as reinforcement in MMC's. Some work has been done in the area of plastically deforming polymers, alone and within a flexible polymer matrix, and the properties of polymers such as polyethylene, polypropylene, poly(arlyletherketone), and polyether-etherketone (PEEK) have been substantially improved by extruding and drawing. Nearly a three-fold increase in modulus for PEEK has been observed after drawing it through a die to a reduction of 3:1 at 310.degree. C. (A. Richardson et al., Polymer Engineering and Science, 25(6)(1985), 355-361). It has also been found that under certain conditions thermotropic liquid crystal polymers form high-modulus and high-strength filaments when deformed in a flexible polymer matrix at high strain rates. (A. I. Isayev et al., Polymer Composites, 8(3)(1987), 158-175.) The similarities in flow characteristics between the polymer matrix and the polymer reinforcement material make them compatible for co-extrusion. In other words, their strength and their elastic, viscoelastic, and plastic behavior as a function of temperature and strain rate are similar, causing them to deform similarly during extrusion. However, substantial differences exist between the way polymers and metals flow and deform, making co-processing appear unattractive and difficult.